1. Field of the Invention
The invention relates generally to the field of neutron generator-type well logging instruments. More specifically, the invention relates to structures for high voltage power supplies used with neutron generator-type well logging instruments to enable better placement of internal components of such instruments.
2. Background Art
The characteristics of geological formations are of significant interest in the exploration for, production and monitoring of subsurface water and mineral deposits, such as oil and gas. To that end, a variety of techniques have been developed to measure subsurface characteristics and evaluate the characteristics to determine certain petrophysical properties of interest. These techniques typically include the subsurface deployment of tools or instruments having energy sources to emit energy into the formations (usually from within a borehole traversing the formations). The emitted energy interacts with the surrounding formations to produce signals that are detected and measured by one or more sensors on the instrument. By processing the detected signal data, a profile or log of the subsurface properties is obtained.
A variety of logging techniques have been developed to evaluate subsurface formations. A number of such techniques include emitting neutrons into the formation and evaluating the results of neutron interactions with formation nuclei. Various types of radiation sources have been used in subsurface well logging systems. For example, neutrons or gamma rays may be generated simply through the use of radioactive isotopes (which naturally decay over time), an x-ray source may be used or neutrons may be generated in an electronic device utilizing a nuclear fusion reaction to generate neutrons on demand. In such electronic neutron sources, high-energy neutrons may be generated through the controlled collision of energized particles by using a nuclear fusion reaction caused by accelerating ions toward a target at high voltage, so as to emit neutrons in bursts of fully controllable lengths and time sequences. Such devices may be referred to for convenience as neutron generators to distinguish them from chemical isotope sources. One neutron generator (referred to as a “pulsed” neutron generator) is described in U.S. Pat. No. 3,461,291. The neutron source described in the '291 patent uses an accelerator tube in which charged particles, such as deuterium ions, are accelerated across a high voltage potential and contact a target element such as tritium. The reaction between the deuterium ions with the tritium target produces substantially monoenergetic bursts of neutrons at an energy level of about 14 million electron volts (MeV). In most well logging applications the neutrons are not emitted continuously but in short bursts of well-defined lengths and sequence of repetition, however continuous generation of neutrons is also possible. When using such a neutron generator, the formation surrounding the instrument is typically subjected to repeated, discrete “bursts” of high energy neutrons. U.S. Pat. Nos. 4,501,964, 4,883,956, 4,926,044, 4,937,446, 4,972,082, 5,434,408, 5,105,080, 5,235,185, 5,539,225, and 5,608,215, for example, describe well logging instruments equipped with pulsed neutron generators.
In well logging using a neutron generator, the borehole and surrounding formation are irradiated with neutrons, and the various interactions of the neutrons with constituent nuclei are measured. Pulsed neutron well logging instruments typically include one or more sensors or detectors that record numbers of neutrons, particularly epithermal energy and thermal energy, as well as gamma rays which are emitted as a result of the interaction of the neutrons with the subsurface formations and the fluids in the borehole itself. The gamma rays may include inelastic gamma rays which are a consequence of high-energy collisions of the neutrons with atomic nuclei in the subsurface formations, as well as capture gamma rays emitted when low energy (thermal) neutrons are captured by susceptible atomic nuclei in the formations (for example, chlorine). Various relevant well logging techniques and tools are described, for example, in U.S. Pat. No. 4,390,783 to Grau, U.S. Pat. No. 4,507,554 to Hertzog et al., U.S. Pat. No. 5,021,653 to Roscoe et al., U.S. Pat. No. 5,081,351 to Roscoe et al., U.S. Pat. No. 5,097,123 to Grau et al., U.S. Pat. No. 5,237,594 to Carroll, and U.S. Pat. No. 5,521,378 to Roscoe et al.
Properties of the formations which may be determined as a result of measuring neutron and gamma ray phenomena include, for example, formation density, fractional volume of void or pore space in the formation (porosity), carbon/oxygen (C/O) ratios, formation lithology, and neutron capture cross section (Sigma), among other measurements. Properties which may be determined by spectral analysis of the gamma rays include concentration of various chemical elements, for example. Properties of fluids in the wellbore may also be determined from various neutron and gamma ray measurements.
Nuclear measurements are also useable in nuclear spectroscopy techniques to obtain qualitative and quantitative information related to subsurface fluid movement. U.S. Pat. No. 5,219,518 describes an instrument equipped with a neutron source and sensors adapted to measure water flow through neutron oxygen activation. Alternative techniques for subsurface fluid measurements include the use of radioactive markers or tracers to identify flow path between formations or wells. U.S. Pat. Nos. 5,049,743, 5,182,051, 5,243,190, and 5,929,437 describe the use of elements that can be made radioactive by bombardment with neutrons so their location can be determined by nuclear logging. Logging tools equipped with gamma ray detectors are particularly suited to distinguish and determine the location of trace materials.
The nuclear phenomena detected with the foregoing instruments are representative of interactions not only with the formation nuclei, but also with the instrument and the borehole. In order to penetrate the formation, the high energy neutrons must pass through the fluid in the borehole (and casing in some applications) before entering the formation. The resulting non-formation contributions to the measured radiations significantly complicate the analysis of the formation characteristics. The problem is all the more complex since the sensitivity of the detector(s) to the radiations coming from the borehole, instrument and the formation, is a function of many parameters, such as, to name a few, lithology, porosity, borehole size, casing size/weight/eccentricity, cement quality, detector housings, or borehole fluid composition. In practice, several techniques have been devised to account for these contributions and to discriminate the undesired radiations from the desired radiations.
For certain types of neutron measurements, the neutron generator and its associated high voltage power supply may be disposed in an instrument housing along with neutron detectors, a neutron monitor detector, gamma ray detectors, or some combinations of the foregoing. Neutron generator well logging instruments known in the art include a high voltage power supply to operate the neutron generator disposed adjacent to the neutron generator. Certain of the foregoing detectors may need to be in nearly identical longitudinal positions as the neutron generator within the instrument in order to optimize the measurements made by the such detectors. Such optimal detector placement is impracticable using conventional instrument configurations where the neutron generator and its associated high voltage power supply are disposed proximate each other. FIG. 1 illustrates such conventional instrument configuration. The instrument 10 includes a spectroscopy detector 12 operable for gamma ray detection, neutron detectors at different longitudinal positions, e.g., near 14 and far/array 16, a pulsed neutron generator 18, and appropriate shielding 20.
FIG. 2 illustrates another known instrument configuration. The instrument 30 is configured in what is called the “split-physics design”, in which smaller diameter detectors 32, 34 can be disposed longitudinally alongside the neutron generator 36. Larger detectors 38, such as the gamma-ray detector referred to above, can be longitudinally displaced from the neutron generator 36 so that the combination of detectors can be better optimized. Appropriate shielding 39 is also included in the instrument. The instrument shown in FIG. 2 is described, for example, in U.S. Pat. No. 7,148,471 assigned to the assignee of the present invention. Limitations inherent in the configuration of FIG. 2 includes that the size of the detectors 32, 34 near the neutron generator 36 must be relatively small, and/or the diameter of the instrument housing must be relatively large. The foregoing may limit the optimum placement of detectors that are typically larger in diameter, for example, spectral gamma ray detector, or may require the use of instrument housings of such size as to require the use of the instrument only in larger diameter wellbores.
The foregoing statements related to neutron well logging instruments may also apply to well logging instruments having x-ray generators therein.
A need exists for improved radiation generator and detector configuration within a well logging instrument.